Actuating current in CPUs (Central Processing Unit) today tends to increase due to high integration caused by the growth in arithmetic capabilities. And, a leakage-current in a highly integrated CPU has been increasing due to the deterioration in insulation caused by miniaturization of circuit patterns or the like. This also causes the increasing of the current consumption in the CPU. Under the circumstances, semiconductor devices, for use in a power source for driving of devices, which consume a large current of more than 100 amperes have recently been developed.
It is commonly carried out to mount a semiconductor device, in which a CPU (semiconductor chip) is mounted on a device substrate, on a mounting board in a package style that is referred to as BGA (Ball Grid Array) or LGA (Land Grid Array).
A semiconductor device mounted based on the BGA or LGA has a large number of electrode pads arranged in a matrix manner on a rear surface of a device substrate (on a surface opposite to a mounting board). When the electrode pads are connected with corresponding predetermined electrode pads on the mounting board of a drive unit of the semiconductor device, it is possible to supply the power, connect to GND, and input/output processing signals Single-layered or multi-layered printed circuit board has been used conventionally as the mounting board. Wirings and electrode pads for actuating power supply, for GND, and for signal input/output are respectively formed on a wiring layer.
However, in an arrangement in which the wirings and the electrode pads for the actuating power supply, GND, and signal input-output are formed on a single wiring layer of the printed circuit board, the following problem arises. More specifically, the problem that a large current as operating power supply may not be supplied to the semiconductor device (high-power device) whose current consumption is large. This is because the conductor (copper foil) on the wiring layer has a thickness of 30 to 75 micrometers. This causes the problem that the printed circuit board itself generates heat, and/or is unable to follow the change in current consumption caused by the change in the operating conditions of the semiconductor device. This is because a conductor cross sectional area suitable for the driving current is not secured, and because a power loss increases proportionally as the square of the current value with respect to the conductor resistance.
An approach to provide a large current to a high-power device is that the printed circuit board (the mounting board) is formed in a multilayer manner, and a plurality of layers are exclusively used for the power supply and GND. In addition, unexamined Japanese patent publication No. 4-118984 (publication date: Apr. 20, 1992) discloses a mounting arrangement in which a wiring layer for signals and a power supply layer are provided in separate printed circuit boards, and in which the thickness of a conductor in the printed circuit board possessing the power supply layer is thickened.
Conventional high-power devices are not highly integrated, and they supply as power sources a current of not more than approximately 30 amperes. Actually, it is possible to supply current of not more than approximately 30 amperes by the conventional arrangement in which the printed circuit board (the mounting board) is formed in a multilayer manner, and a plurality of layers are exclusively used for the power supply and GND. However, the problem arises that a semiconductor device, which has been recently developed and consumes a large current of over 100 amperes, can not supply appropriate current even if the above conventional arrangement is adopted.
Thus, when supplying a large current of over 100 amperes to a semiconductor device, it is not possible to solve a problem such as the heat generation within the printed circuit board. This is because the power loss increases due to the thin conductor of not more than 70 micrometers in the printed circuit board. If it is intended to solve the problem of the current supply by making the printed circuit board to have more layers, then the cost of the printed circuit board will dramatically increase.
Moreover, the operating voltage of such a high-power device has been made lower these days to reduce the power consumption and/or the leakage current. This results in that a drop of the voltage to be supplied to the semiconductor device due to the resistance loss of the conductor is no longer tolerated.
Further, before shipment of a semiconductor device, it receives two kinds of tests. One is a test called a burn-in for finding initial failure, and the other is a test for distinguishing defectives from non-defectives. In the burn-in, a heavier load than in the ordinary use is supplied to the subject semiconductor device. Consequently, the current supply in the burn-in becomes larger than in the ordinary use. This gives rise to at least the following defects in the drive unit, of a burn-in apparatus, for supplying the actuating current to the semiconductor device. Namely, a DC power supply for compensating the voltage drop becomes bulky, and a cooling device for the heat generated becomes bulky.